Romeo gripped by theatrical muscle pain (5)
I believe the answer is:
cramp
'muscle pain' is the definition.
(I've seen this before)
'romeo gripped by theatrical' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'gripped by' means one lot of letters goes inside another.
'theatrical' becomes 'camp' (I've seen this before).
'r' inserted into 'camp' is 'CRAMP'.
(Other definitions for cramp that I've seen before include "Muscular spasm, very painful" , "Involuntary and painful contraction of a muscle" , "common among writers" , "To restrict; a muscle pain" , "restraint" .)